Angiography is the visualization of blood vessels by use of X-rays. For this purpose, a contrast agent, i.e. a substance, which is barely permeable for X-rays, is injected into the blood vessel. The radiographic image then shows the vessel's inner cavity filled with contrast agent. The resulting image is called angiogram. In medical language, the long word angiography is often shortened by the word angio. Computer tomography can also be used for angiography.
Computer tomography, of which the short form is CT, is the computer-assisted analysis of a number of radiographic images of an object, taken from different directions, in order to produce a three-dimensional image (voxel data). This is an imaging method.
The techniques currently used for image-producing vessel examination can be divided into three groups of methods: On the one hand casting techniques are available, on the other hand contrast agents exist, which are able to penetrate capillaries. A general overview of all techniques of postmortem angiography can be found in “Postmortem Angiography—A Review of Former and Current Methods” (S. Grabherr et al, AJR 2006 in press).
Casting Techniques:
For the casting techniques, for example methylmethacrylate (for example MERCOX® (methyl metharcrylate monomer solution (acrylic resin mixture)) or MICROFIL® (silicone rubber injection compounds), are used. These substances are injected, immediately after their mixing, after the vessels have been rinsed. After their hardening the tissue is macerated, which takes 2-4 weeks. What is then left over is the cast of the vessels, which can be analyzed by the naked eye, with the aid of a microscope or electron microscope, or also by radiographic techniques (see Djonov et al., Anal Embryo 2000; 202:347-357). Disadvantages of this method are the tedious manufacture of these preparations, the long time necessary for maceration and nonetheless the high risk of damaging the preparations by maceration and the subsequent handling, during which small parts of the preparation can easily break off.
Liquid Contrast Agents:
Most of the liquid contrast agents are not suitable for microangiographies. Water-based mixtures have the property to very quickly exit the vessel by penetrating the vessel's wall (so-called extravasation), which leads to diffuse contours of the represented vessels. Furthermore, their contrast is not high enough to clearly represent small vessels, such as capillaries.
Oily contrast agents have the property to remain intravasal without penetrating the vessel's wall (normally no extravasation). This leads to a clearly contoured image of the vessles. However, capillaries cannot be penetrated due to possible occlusions of the so-called capillaries [hair vessels].
Corpuscular contrast agents can be used for microangiography if small corpuscular particles are used. The most common corpuscular agent is MICROPAQUE® (barium sulfate-containing x-ray contrast agent). However, in its use in combination with micro-CT-devices, the problem of precipitation of the dissolved particles was described, which leads to artifacts (M. Marxen et al., Med Phys 2004, 31:305-313). Marxen clearly describes in his paper the necessity and the lack of a suitable contrast agent for microangiography using micro-CT.